As having high electromotive force and high energy density, a lithium battery is widely used in a field of an information technology equipment and a communication device. Meanwhile, in an automotive field, there is a strong need for development of an electric vehicle or a hybrid vehicle due to an environmental problem and a resource problem. In this regard, a lithium battery is also considered as a power source for them. The lithium battery generally comprises a cathode active material layer which contains a cathode active material, an anode active material layer which contains an anode active material, and an electrolyte layer that is formed between the cathode active material layer and the anode active material layer.
Conventionally, carbon materials (for example, graphite) are used as an anode active material of a lithium battery. Meanwhile, to further improve safety, an active material with higher thermal stability is required. In this regard, a non-aqueous electrolyte battery in which lithium titanate (LTO) is used as an anode active material is disclosed in Patent Literature 1. Since LTO is an oxide, it has high thermal stability, and thus it is advantageous in terms of safety.
However, Li insertion and extraction electric potential of LTO (oxidation and reduction electric potential) relative to Li metal is about 1.5 V, which is higher than conventional carbon materials (about 0.3 V). Thus, battery voltage of a battery using it is low. Since the battery voltage can be defined by a difference between Li insertion and extraction electric potential of a cathode active material and Li insertion and extraction electric potential of an anode active material, for example, there is a problem that the battery voltage is lowered when Li insertion and extraction electric potential of an anode active material is increased under the condition in which the same cathode active material is used.
In Non Patent Literature 1, evaluation of the physical properties of LixY2Ti2O5S2 is disclosed. However, according to the literature, only the evaluation of physical properties of LixY2Ti2O5S2 is made and absolutely no evaluation is made on characteristics of a battery. Further, according to Non Patent Literature 1, as a method of synthesizing LixY2Ti2O5S2, Li is forced to be incorporated to Y2Ti2O5S2. However, there is no description suggesting that the Li may be extracted, and naturally no description suggesting that it may function as an active material is included therein. In Non Patent Literature 2, evaluation of the physical properties of Nd2Ti2O5S2 is disclosed. However, according to the literature, only the evaluation of physical properties of Nd2Ti2O5S2 is made and absolutely no evaluation is made on characteristics of a battery.